Influence of Fragmentor Voltage and Solvent on Negative Ionization Behaviors of Uvinul 3039 Using LC/APCI-MS

Abstract

Engineering plastics undergo degradation when exposed to the ultraviolet radiation in sunlight. This degradation process results in appearance changes such as discoloration, chalking, and crazing as well as reduction of physical properties. UV absorbers slow down the degradation process by absorbing harmful ultraviolet radiation and dissipating it as thermal energy. Analysis of UV absorbers is very important to understand polymer degradation studies and qualityassurance testing. 2-Ethylhexyl-2-cyano-3,3-diphenylacrylate (Uvinul 3039) is one of popular cyanoacrylate UV absorbers since it is aprotic and stable UV absorber. Uvinul 3039 has a single absorption band with a maximum at 305 nm. Analysis of UV absorbers is difficult because of requirement of a sensitive method for identification, high molecular weight, and high polarity. Thus, gas chromatography (GC) is not suitable for analysis of UV absorbers. Although high performance liquid chromatography (HPLC) with UV detection has been successfully used, this method is only applicable to compounds having UV-absorbable groups. These problems can be solved by the use of liquid chromatography/mass spectrometry (LC/MS), 1,2 especially LC/MS with atmospheric pressure chemical ionization (LC/APCIMS) 3 . Block and coworkers 3 analyzed antioxidants and UV stabilizers for polymeric materials using LC/APCI-MS. In APCI, a corona discharge makes solvent reactant ions that are used to ionize the sample by chemical ionization. Solute and sample are vaporized by pneumatic nebulization. APCI relies upon gas-phase ion-molecule reactions to place a charge on neutral analytes, so it is especially important to understand these reactions. Kostiainen and Bruins reported that a judicious selection of solvent can be made to promote ionization of the compound of interest. 4-7 When mobile phase transports analyte molecules, APCI can deliver a proton to the analyte. The solvent-derived ions serve as reagents for proton transfer ionization. Mass spectrometry with APCI gives a great deal of structural information in a short analysis time. In APCI-MS, fragmentation pattern can be obtained by varying the fragmentor voltage (or accelerating voltage) and some fragment ions are produced to identify the structure of an analyte. 8 A key parameter in tuning the instrument’s sensitivity is the fragmentor voltage (similar to cone voltage in other instruments). At higher fragmentor voltage settings, collisioninduced dissociation (CID) can be initiated in the region between the end of the transfer capillary and the first skimmer cone, so that fragmentation increases. 9 In this study, we analyzed Uvinul 3039 using LC/APCI-MS with negative ion mode and investigated the ionization behaviors such as formation of solvent-adducted ion and fragmentation by varying the fragmentor voltage and kind of solvent. Methanol, ethanol, and acetone were employed as the solvent and eluent. These solvents have been used for extraction of organic additives including UV absorbers from plastic polymers such as polyethylene (PE), polypropylene (PP), poly(ethylene terephthalate) (PET), and poly(vinyl